Field of the Invention
The present invention relates to an optical scanning apparatus included in an image forming apparatus, such as a laser beam printer (LBP), a digital copying machine, and a multifunction printer (MFP).
Description of the Related Art
As an optical scanning apparatus included in an image forming apparatus, an optical scanning apparatus is known which can optically scan photosensitive surfaces (scanned surfaces) of a plurality of photosensitive members at the same time by a single deflector. A patent literature (Japanese Patent Application Laid-Open No. 2013-64857) describes an optical scanning apparatus which is downsized by disposing a mirror on each optical path from a deflector to each scanned surface to bend each optical path. A patent literature 2 (Japanese Patent Application Laid-Open No. 2014-48563) describes an optical scanning apparatus which adopts a multistage lens in which a plurality of lens surfaces is combined by overlapping with each other in a sub-scanning direction to reduce the number of lenses. A patent literature 3 (Japanese Patent Application Laid-Open No. 2007-155838) describes an optical scanning apparatus which devises arrangement of mirrors for reflecting a light beam passed through a multistage lens to reduce the number of mirrors.
However, in the optical scanning apparatuses described in the patent literatures 1 to 3, optical path lengths with respect to each scanned surface are the same with each other, so that a degree of freedom in arrangement of each optical component is low, and it is difficult to avoid interference of a light beam with an imaging lens while downsizing the entire apparatus.
According to the optical scanning apparatus described in the patent literature 1, the imaging lens is arranged between two mirrors on an optical path corresponding to a photosensitive member closer to the deflector, and if each component is arranged so as not to interfere the light beam, it is difficult to further thin the entire apparatus. According to the optical scanning apparatus described in the patent literature 2, three mirrors are arranged on an optical path corresponding to the photosensitive member closer to the deflector to avoid interference of the multistage lens with the light beam, and it is difficult to reduce the number of components in the entire apparatus. According to the optical scanning apparatus described in the patent literature 3, a large reflection angle of a mirror is set on an optical path corresponding to the photosensitive member closer to the deflector to avoid interference of the multistage lens with the light beam, so that a distance between two mirrors is long, and the apparatus becomes large.